This invention relates generally to a bimetal actuated lock for a lid or door and more particularly is directed to a lock for the lid or door of a laundry appliance such as a washing machine where the lock is actuated automatically when the machine is in operation to prevent opening of the lid or door while the spin tub is rotating at a high rate of speed.
Washing machines, whether of the top loading or front loading type, have two general modes of operation. During the wash mode, the movement of the parts is generally at a rather slow speed to cause the relative movement of the clothes and the wash liquid, and at this speed no particular danger is likely to result if the lid or door were opened and the operator were to place a hand or other object inside the machine, since such machines have an electrical interlock which de-energizes the motor whenever the door or lid is opened. However, when such machines are in the spin cycle in which as much liquid is removed from the clothes as possible, the spin tub is rotating at a high rate of speed and with the load of wet clothes represents a relatively large magnitude of angular momentum. Under these conditions, if the lid or door is opened, the operation of an interlock switch to de-energize the motor is insufficient to bring the parts to a stop quickly and if a person were to reach into the machine under those conditions, severe injuries could result.
In view of this problem, two approaches have been used as a safety feature. One of these is to utilize a brake in the drive mechanism which is automatically actuated upon de-energization of the motor by the interlock to apply a positive braking action and bring the rotating spin tub to a halt as quickly as possible. However, the brake mechanism is rather expensive and subject to wear after an extended period of time, which tends to decrease its effectiveness and cause a longer period of time to elapse before the spin tub is completely stopped.
The other approach to solving this problem is to use a locking device to positively prevent the opening of the lid or door of the washing machine whenever the spin tub is rotating. This can be accomplished by the use of a solenoid-operated lock which positively engages the door as long as the solenoid is energized and then releases the door after the tub is stopped to allow access to the interior of the washing machine. It has been recognized that a solenoid-type locking device has shortcomings not only because of the high cost and reliability question of the solenoid itself but also the need to provide extra contacts on the timer to provide the necessary time delay after the motor has been de-energized for the spinning tub to come to a stop.
An alternative to the solenoid locking device has been proposed in the form of a lock which is actuated by a bimetallic element which is heated by the current passing through the drive motor. Such a device has been disclosed in the present inventor's U.S. Pat. No. 4,074,545 granted Feb. 21, 1978. A similar device is also shown in U. S. Pat. Nos. 4,179,907 and 4,286,811. With this locking arrangement, the high inrush current when the motor is energized causes a large current to flow through the bimetal element to rapidly heat it so that the latch blade will move rapidly into a locking condition. When the motor is de-energized, the thermal lag in such a device causes the locking blade to remain in the engaged position for a period of time for the bimetal element to cool off and the parameters of operation can be adjusted so that the blade remains in the locking position for a sufficient period of time for the spin basket to come to a complete stop. However, it has been recognized with this prior bimetal lock, that the parameters must be chosen such that it will actuate and heat to a temperature sufficiently high that cooling will take a long enough time that the lock will not disengage before the spin tub stops under any load conditions. Thus, the lock must be designed to work with a minimum load, and when the machine is used with a heavy load, particularly one that has a relatively high imbalance, the increased motor torque and hence current through the lock will tend to noticeably increase the unlocking time.